In a company specializing in creating color printed materials, such as a printing company, in order to maintain quality of printed materials, colors of a printed material are measured to adjust colors of a printing unit which has printed the printed material, on a regular basis. In such color adjustment for a printing unit, as one example, original image data, called “color chart”, is printed by the printing unit, and respective colors of a plurality of patches of the printed color chart are measured by a color measurement device. Then, an amount of color deviation between an actual measured value and a target value of a color of each patch is evaluated, and, according to a result of the evaluation, colors of the printing unit are adjusted.
The color chart is constructed such that it comprises a plurality of color samples, called “patches”, as mentioned above, wherein each of the patches are formed differently in terms of color (hue, brightness (luminosity), chroma (colorfulness, saturation)), and arranged in a given manner. Such a color chart includes various types. For example, there is one type of color chart constructed such that a plurality of quadrangular-shaped patches having various colors are arranged in horizontal and vertical directions in a two-dimensional array configuration. In this type of color chart, depending on intended contents of the evaluation, there are various patterns, such as a pattern in which the patches are arranged to form a random (arbitrary) color array, and a pattern in which the patches are arranged such that a change in shade between adjacent ones of the patches becomes smaller, like a gradation. This type of color chart includes not only a color chart which is created by a user using a color chart creation tool provided from a manufacturer of color measurement devices, but also a color chart which is provided from a public agency. As above, a color chart can have a significantly wide variety of patterns by differences in shape, arrangement, color combination and others of the patches.
Meanwhile, the number of colors for use in color adjustment for a printing unit has been increasing year after year. Accordingly, the number of patches arranged in a color chart has also been increasing, wherein a size (area) of each patch is relatively small.
From such a circumstance, it has become practically impossible to manually accurately adjust a position of a measuring section of a color measurement device with respect to each patch, so as to perform a color measurement. For this reason, there is a need for an automatic system for automatically measuring a position of each patch, and automatically adjusting a position of the measuring section of the color measurement device to become coincident with the measured position of the patch, so as to measure a color of the patch. As one example of this system, Gretag-Macbeth AG proposed a method which comprises: taking a two-dimensional color image of a color chart to be measured; calculating a position of each patch by an image processing technique using a computer; and moving a color measuring head to the determined position of the patch so as to measure colors of the color chart, as described in the following Patent Literature 1.
In this regard, for deriving a position of each patch by the above image processing, there is a need for an imaging unit such as a camera for taking an image of a color chart, in addition to a color measuring unit such as a colorimeter for measuring a color of each patch of the color chart. The imaging unit and the color measuring unit are separate units, and thereby arranged at separate positions, individually. Thus, for moving the color measuring unit to a position of each patch derived based on the image of the color chart obtained by the imaging unit, there is a need for information about a relative position relationship between the imaging unit and the color measuring unit.
As for this information about the relative position relationship, it is conceivable to preliminarily store it in a color measurement device, based on design values of the color measurement device. However, in an actual device, there are an installation error and a dimensional accuracy error in each of the imaging unit and the color measuring unit. Thus, the use of design values is likely to lead to a situation where the color measuring unit cannot be always moved to a position of each patch derived based on the image of the color chart obtained by the imaging unit. Moreover, when a target one of the patches to be measured is adjusted to be positioned at the color measuring unit while feeding the color chart by a conveyance unit such as a conveyance roller, slip occurring between the color chart and the conveyance roller is likely to lead to a situation where the color measuring unit cannot be moved to a position of each patch derived based on the image of the color chart obtained by the imaging unit.